With some concrete construction, especially in high-traffic areas, it is desirable to have the affected areas in service as soon as possible. Typically, a minimum compressive and/or flexural strength is specified, and must be reached, before opening the area to use. In congested urban areas, traffic volumes can be so high that the contracts often limit shut-down periods for pavement repairs to 8 hours or less. As the removal of old concrete and the required preparation work can be up to 4 hours, the fresh concrete being placed ideally should achieve the minimum specified strength in a maximum of 4 hours after placement. This trend is reflected by the various State Departments of Transportation in their standards for concrete placement, wherein a minimal amount of time is allowed for sufficient strength generation.
Fast-track paving can and does occur with ordinary portland cement (OPC)-based concrete; however, OPC-based concrete mixtures generally cannot achieve early-strength requirements without sacrificing necessary working, placement and finishing time properties. Portland cement-based concrete mixtures usually require a minimum of twenty-four hours and frequently five to fourteen days to gain sufficient strength to allow a return to service. Roughly a decade ago, fast track OPC-based concrete mixtures could be produced that developed sufficient strength to open a pavement to traffic in less than 12 hours. During the past decade, there has been an increasing emphasis on researching materials and processes that will allow early opening for concrete pavements. For example, in 1998 the Virginia DOT used a rapid strength OPC-based concrete mixture to return the Braddock Road Bridge over Interstate 495 back to service in just under eight hours. In the mid-90's the New Jersey DOT developed a “fast track mix” that could achieve compressive strength of 3000 psi (20.7 MPa) and flexural strength of 350 psi (2.4 MPa) in six hours2.
In the year 2000, Caltrans partnered with the American Concrete Pavement Association to “invite ready-mixed producers and admixture manufacturers to submit concrete mixes that were both cost-effective and fast setting.” Consequently, an experimental program was launched to solve the puzzle of developing high-early flexural strength, such as 400 psi (2.8 MPa) in four hours after placement, using OPC-based concrete without the negative side effects of the proprietary binder systems.
In current practice, when fast turn-around was required, engineers have utilized fast-setting hydraulic cement concrete (FSHCC). For example, when there are very stringent engineering requirements, such as 400-psi flexural strength in four hours after placement, non-portland cement-based materials are used. These materials are very expensive. Additionally, the concrete produced with these binders is also prone to undesirable side effects such as insufficient control of working time, and often requires a follow-up grinding process to achieve an acceptably smooth surface.
To achieve a rapid level of strength development without an external heat source, it is most unlikely that one could be successful using traditional dispersant chemistries due to their excessive retarding effect on cement hydration.
U.S. Pat. No. 5,494,516 discloses a process for modifying the slump of a concrete or mortar by the addition at different times of a water-soluble poly(alkylene oxide) and a β-naphthalene sulphonate-formaldehyde condensate, a plasticizer or superplasticizer.
U.S. Pat. No. 5,792,252 discloses a process producing a cementitious composition that has a set time which is initially retarded for extended workability, followed by accelerated hardening comprising adding to a cementitious composition an admixture of a) an alkali metal carbonate and b) a mono- or di-carboxylic acid which is used as an accelerator.
What is needed in the industry is an affordable cementitious composition that provides acceptable and predictable workability while exhibiting high early compressive and flexural strength using conventional and locally available materials.